In a charged particle beam apparatus typical of which is a scanning electron microscope, desired information (for example, a sample image) is obtained from a sample by scanning a thinly converged charged particle beam on the sample. In such a charged particle beam apparatus, the resolution becomes higher year by year, and the required observation magnification becomes higher as the resolution becomes higher. As the beam scanning method for obtaining a sample image, there are a method which obtains a final objective image by adding a plurality of images obtained by high speed scanning and a method which obtains a final objective image by once of low speed scanning (acquiring time of one frame image: approximately 40 seconds to 80 seconds). The influence of the drift of a view area on the acquired image becomes more serious as the observation magnification becomes higher. For example, in the method of acquiring the objective image by adding image signals obtained by the high speed scanning pixel by pixel (frame addition), when there is drift caused by charge-up of the sample during adding the images, the objective image after adding has blurs in a direction of the drift because displaced pixels of the view area are added. Reducing the influence of the drift may be attained by reducing number of adding frames and shortening the adding time, but this method can not obtain a sufficient S/N ratio.
On the other hand, in the method of acquiring the image by the low speed scanning, when there is drift during acquiring the image, the image is deformed because the view area flows in a direction of the drift.
A technology is disclosed in Japanese Patent Application Laid-Open No. 62-43050. The technology is that a pattern for detecting drift is stored, and a beam irradiating position is corrected by periodically acquiring an image of the pattern to detect a displacement between the acquired image and the stored pattern.
A technology is disclosed in Japanese Patent Application Laid-Open No. 5-290787. The technology is that two images are acquired based on electron beam scanning on a specified observed area, and pattern matching is performed in order to specify an amount of displacement and a direction of displacement between the both images, and pixels are added by moving the pixels by the specified amount of displacement and the specified direction of displacement.
In the technology disclosed in Japanese Patent Application Laid-Open No. 62-43050, the accuracy of controlling the beam irradiating position becomes insufficient when the observation magnification becomes several hundred thousand times. For example, when an image of 1280×960 pixels is tried to be acquired with an observation magnification of 200 thousand times, the size of one pixel on the observation view area (on the sample) is approximately 0.5 nm. Measurement and evaluation with a higher magnification become necessary as the scale-down of a measured object is progressed. Under such a condition, when the technology is applied to an apparatus for forming a final image by adding a plurality of images, image shift (drift) below several nm causes “blurs” in a flame added image.
Although the technology disclosed in Japanese Patent Application Laid-Open No. 62-43050 suppresses the image shift by controlling the scanning position of the electron beam to correct the drift, the correcting accuracy of the position by such control is limited to several nm to several tens nm. Accordingly, it is almost impossible to correct the position (correct the drift) of an image having a magnification the position above several hundred thousand times with a pixel level. In addition, there is a problem in that the through-put is decreased because stabilization of the drift takes a long time.
On the other hand, the technology disclosed in Japanese Patent Application Laid-Open No. 5-290787 can be appreciated in the point that the position between the images can be corrected in the pixel level, but there is the following problem.